Just copypaste it into the urlbar on any page you want to run it on. I think it might also be possible to store it as a bookmarklet, by making a new bookmark with that as the URL.

I did some quick checking and turns out I’m not the first guy to think of this, but the scripts I found none worked universally in more than just FF or IE. I tested my script in IE7, Opera 9 and Firefox 3, and it appears to work in all of them, and quite likely works in every other browser that can run it.